1. Field Of The Invention
This invention relates generally to the field of processing of materials, such as uncured rubber pellets, which have sticky surface properties. More particularly, the invention relates to anti-stick coating compositions useful as a parting of detackifying agent for preventing solid articles having sticky surface properties from sticking to one another during the processing of the same.
2. Description Of The Prior Art
Many synthetic and naturally occurring polymeric elastomeric materials are manufactured in an uncured stage for subsequent use in the manufacture of products such as tubing, belts, tires, rubber sheeting and the like. In the production of the final product, the uncured materials are subjected to curing or vulcanization in an appropriate molding apparatus or press. The handling of such uncured materials in mechanical processing equipment and the storage thereof is extremely difficult because of the tackiness, or stickiness, of the materials in their uncured stage. Accordingly, many attempts have been made, in the past, to develop some means for rendering the uncured articles less tacky without substantially interfering with the subsequent curing or vulcanizing operations and without substantially altering the physical characteristics of the resultant product.
One method previously attempted consisted of applying a particulate material such as talc, starch, cellulose fibers, vinyl aromatic polymer dust, or hydrophobic silica powder directly to the uncured pellets or articles. The principal difficulties involved in such a procedure is that it is practically impossible to apply an even coating of the material, and dusting problems are severe. This latter factor of course presents two problems, one being the safety hazard and the other being the excess cost of wasted materials.
Another prior art method for detackifying uncured rubber stock and the like comprised the incorporation directly into the rubber composition of an internal lubricating material. Of course it will readily be apparent to one of ordinary skill in the art to which the present invention pertains that such a method would involve additional processing steps and the use of relatively expensive materials. Moreover, it would be necessary for the lubricating material to be dispersed throughout the entirety of the uncured rubber pellets and accordingly, the total quantity present at the time of cure would be much greater than in the case of an external coating. Moreover, the problems involved in selecting a compatible material which could be utilized as a lubricant but which would not affect the final properties of the cured article are manifest. Lubricated materials also are difficult to handle in some processing apparatuses, such as, for example, vertical screw conveyors.
Yet another prior art method for detackifying uncured pellets comprised the coating of each pellet with a continuous outer film of a hard, non-tacky material such as polyethylene or the like. Again, the obvious shortcomings are apparent because such procedure would involve expensive processing steps and the compatibility or lack thereof of the coating film with the rubber compound would add another variable to subsequent processing operations.
A known prior art liquid anti-tack composition which utilizes bentonite clay is disclosed in U.S. Pat. No. 3,765,911 which issued on Oct. 16, 1973 to Knowles et al. The Knowles et al. composition is described as necessarily including a strong gelling grade of bentonite as opposed to ordinary or non-gelling grades of this material. Moreover, the Knowles et al. compositions also necessarily include either a water soluble salt of an alkyl ether sulfate or a water soluble salt of an alkyl alcohol sulfate. In each of the specific compositions described in the Knowles et al. disclosure, the quantity of soap is much greater than the overall quantity of bentonite. In particular, the utility of the Knowles et al. compositions is extremely limited because of this necessity for inclusion of a strong gelling grade of bentonite coupled with a large proportion of soap with respect to the amount of bentonite present and this latter factor contributes to the necessity for using slurries containing relatively low concentrations of the bentonite component. Even in Example V of Knowles et al., the total amount of clay in the slurry amounts to only approximately 6.6 times the amount of pale oleine oil saponified to produce the soap component. Moreover, the composition exemplified in Example V of the Knowles et al. disclosure also includes large relative quantities of sodium/ether sulfate (anhydrous) and sodium nitrite. al. the exact basis for the inclusion of these latter two components is not particularly clarified in the Knowles et al disclosure, it seems reasonable to assume that they were included for the purpose of providing a composition having operable physical properties.
It is also to be noted that the compositions exemplified by Knowles et al. are described as being in a concentrate form and substantial dilution with water is intended. At column 6, line 26, Knowles et al. suggest a 10-fold dilution as resulting in loss of stability of the colloidial dispersion; however, there is no disclosure at all with respect to exactly how much dilution is permissible. Accordingly, there really is no way to tell from the Knowles et al. disclosure just how much clay is intended in the composition as the same is applied to articles to be detackified.